1. Field of the Invention
The present invention relates to tire treads based on polydiene rubbers.
2. Discussion of the Related Art
The tire treads occupy a special place among the structural components of the tire. This is due in particular to the fact that the tire tread forms the narrow contact surface between vehicle and roadway. The characteristics of a vehicle during driving depend in particular on the type and equality of the tire tread. An optimum tire tread must cover an extensive requirement profile. In addition to high abrasion resistance and all-weather properties, tire tread should exhibit good winter properties and low rolling resistance. Antiskid properties, in particular on a wet roadway, are very important from the point of view of driving safety. In addition to the conventional braking with locking, the antilock braking system (ABS), which is increasingly improved and widely used in recent years, is becoming increasingly important also for the development of new tread polymers, which must be specially tailored to the particular conditions during ABS braking. EP-A 0 430 617 and EP-A 0 500 338 describe tread rubbers for improved ABS braking, containing the conventional aromatic oils as plasticizers.
The room temperature elasticity is of key importance for assessing the wet skid behavior on a laboratory scale. It has in the past proven useful for evaluating the wet skid behavior in the case of emulsion SBR.
In order to achieve good braking values, it is necessary to use tread polymers having high damping, i.e. having a particularly low room temperature elasticity. This generally requires the preparation of polymers having a high glass transition temperature according to DE-A-37 24 871. These polymers are unsaturated, elastomeric AB block copolymers comprising
from 40 to 80% of a block A based on butadiene (having a uniformly distributed vinyl group content of from 8 to 60%),
from 60 to 20% of a block B containing:
up to 60% of butadiene, PA1 from 0 to 60% of isoprene and PA1 from 0 to 45% of styrene, PA1 from 10 to 60 phr of polybutadiene rubber and PA1 from 90 to 40 phr of copolymer. PA1 the tetrahalides of the elements Si, Ge, Sn and Pb, in particular SiCl.sub.5 ; PA1 organic compounds of the general formula R.sub.n [SiHal.sub.3 ].sub.n, where "Hal" denotes a halogen and where n=1 to 6, in particular n=1 or 2; and where R is an n-valent organic radical, for example an aliphatic, cycloaliphatic or aromatic radical having 6 to 16 carbon atoms; 1,2,4-tris(2-trichiorosilyiethyl)cyclohexane, 1,8bis(trichlorosilyl)octane and 1-(trichlorosilyl)octane may be mentioned by way of example; PA1 SiHal.sub.2, such as, for example, dimethylsilyl chloride; PA1 halosilanes of the general formula Si(H).sub.m (Hal).sub.4-m where 3.gtoreq.m&gt;1; and PA1 di- and trivinylbenzenes, such as, for example, 1,4-divinylbenzene.
and where the vinyl content of the diene units is from 75 to 90%.
All percentages are expressed in weight percent unless otherwise specified.
Copolymers prepared by anionic polymerization and comprising butadiene and styrene are also suitable. These copolymers (referred to below as solution SBR) consist of a mixture of butadiene and from 15 to 35% of styrene, where the vinyl content of the butadiene units (based on butadiene) is between 35 and 90%.
These block copolymers (integral rubber) or solution SBR have a high glass transition temperature.
In Table 1, this is realized in the case of Example 2 using a typical AB block copolymer B as a model. Compared with the base (Example 1), values of 101 and 116 are obtained during braking with blocking on asphalt and concrete, with an elasticity of 8. However, the decrease in the case of ABS braking on asphalt to 85% compared with the base is completely surprising. This means that the wet skid potential incorporated in the integral rubber molecule cannot be realized on the road under slight stresses as represented by ABS braking on asphalt at low speeds.
During braking on concrete, i.e. with higher stress, virtually the standard level is achieved with a rating of 98%.